generate-reprepro-codename: Make architectures and (udeb) components configable.

Architectures, components and udeb components are now configurable by either
using parameters or environment variables. Parameters will overwrite
environment variables.

The udeb components parameter value may be explicitly(!) empty (e.g.
--udeb-components "") in which case UDebComponents is not set in the created
distribution. If no parameters or environment variables are given, sane
defaults are used.